From 034dd340b08be1f2f0477ad16131d609f9dbd53c Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sun, 30 Aug 2026 09:22:00 -0700 Subject: Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace Pull tracing fixes from Steven Rostedt: - Fix error output of boot instance creation failure Currently if a boot instance creation fails, instead of printing out the name of the instance that failed, it prints "(null)". That is because it prints "cur_str" that had already been processed by strsep(). Print the saved name instead. While at it, print the error code of the failure. - Fix use-after-free for same named historgrams Histograms can be named so that they can be used in multiple events. But if the named histogram has a variable attached, the second event that uses the named histogram which duplicates it and needs to free the original after duplication leaves the old variable in place and still visible. If another histogram uses than variable, it will use the stale one which will try to reference the freed duplicate histogram and crash the kernel. Free the duplicate variables along with the duplicated histogram data. - Check return value of kthread_run() in event self test The events self tests uses a kthread for testing but does not check if it succeeded in creating a kthread. If the kthread creation were to fail, the code will still try to call kthread_stop() on the error returned. - Fix race between reading trace_pipe and updating subbuffer size If a user is reading the trace_pipe file at the same time they update the ring buffer sub-buffer size, can cause the trace_pipe read to read stale data. Add trace_access_lock() around updating the ring buffer sub-buffer size. - Fix eventfs_inode on failure path in creation of the events directory In the creation of the "events" directory, if after allocating the eventfs_inode a failure is detected, it calls cleanup_ei() which calls free_ei(). The free_ei() will test if eventfs_inode being freed has no children. It is a bug if it does. But on the failure case of the creation of the "events" directory, the children lists have not yet been initialized and the free will trigger a warning because list_empty() on an uninitialized list returns false. Move the initialization into init_ei() where it makes more sense and makes sure that a created eventfs_inode has its lists initialized upon creation. - Check return value of kthread_run() in ftrace direct sample code The sample code that shows how to use the ftrace direct calls does not test the return of kthread_run() to see if it succeeds. Return a failure if the kthread_run() doesn't succeed. - Clear user events state on fork in case of alloc failure On fork, the child gets a pointer to the parent's user events state. It makes a copy of it then updates the child's pointer to it. But if the allocation fails, the duplication function leaves the child with a pointer to its parent's descriptor. When the child cleans up its data, it will free the parent's descriptor while the parent is still using it. In the duplication function, set the child's user_event_mm to NULL before testing if the allocation succeeded, and when it exits it will not free the parent's descriptor. - Fix retry exhaustion in simple ring buffer reader swap simple_ring_buffer_swap_reader_page() starts with retry set to 8 and post-decrements it only after a failed link replacement. On the final attempt, a successful replacement leaves retry at zero, while a failed replacement leaves it at -1. But the check for success expects the retry value to be non-zero and exits with an error on zero. This is the opposite result. Fix it. - Fail nicely when the remote swap_reader_page() returns an error Currently, if the swap_reader_page() of a remote buffer fails, it triggers a WARN_ON_ONCE() and continues normally. Instead, have it exit with an error and a pr_warn() print instead of a full WARNING. * tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: ring-buffer: Stop remote reader update when page swap fails tracing: Fix retry exhaustion in simple ring buffer reader swap tracing/user_events: Clear copied tracing state before fork duplication samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify eventfs: Initialize ei->children and ei->list in init_ei() tracing: Fix use-after-free in trace_pipe read on sub-buffer order change tracing: Fix crash passing ERR_PTR to kthread_stop() tracing: Fix use-after-free with same-name named triggers tracing: Fix logged instance name on creation failure --- Documentation/driver-api/usb/dma.rst | 110 +++++++++++++++++++++++++++++++++++ 1 file changed, 110 insertions(+) create mode 100644 Documentation/driver-api/usb/dma.rst (limited to 'Documentation/driver-api/usb/dma.rst') diff --git a/Documentation/driver-api/usb/dma.rst b/Documentation/driver-api/usb/dma.rst new file mode 100644 index 000000000..02f6825ff --- /dev/null +++ b/Documentation/driver-api/usb/dma.rst @@ -0,0 +1,110 @@ +USB DMA +~~~~~~~ + +In Linux 2.5 kernels (and later), USB device drivers have additional control +over how DMA may be used to perform I/O operations. The APIs are detailed +in the kernel usb programming guide (kerneldoc, from the source code). + +API overview +============ + +The big picture is that USB drivers can continue to ignore most DMA issues, +though they still must provide DMA-ready buffers (see +Documentation/core-api/dma-api-howto.rst). That's how they've worked through +the 2.4 (and earlier) kernels, or they can now be DMA-aware. + +DMA-aware usb drivers: + +- New calls enable DMA-aware drivers, letting them allocate dma buffers and + manage dma mappings for existing dma-ready buffers (see below). + +- URBs have an additional "transfer_dma" field, as well as a transfer_flags + bit saying if it's valid. (Control requests also have "setup_dma", but + drivers must not use it.) + +- "usbcore" will map this DMA address, if a DMA-aware driver didn't do + it first and set ``URB_NO_TRANSFER_DMA_MAP``. HCDs + don't manage dma mappings for URBs. + +- There's a new "generic DMA API", parts of which are usable by USB device + drivers. Never use dma_set_mask() on any USB interface or device; that + would potentially break all devices sharing that bus. + +Eliminating copies +================== + +It's good to avoid making CPUs copy data needlessly. The costs can add up, +and effects like cache-trashing can impose subtle penalties. + +- If you're doing lots of small data transfers from the same buffer all + the time, that can really burn up resources on systems which use an + IOMMU to manage the DMA mappings. It can cost MUCH more to set up and + tear down the IOMMU mappings with each request than perform the I/O! + + For those specific cases, USB has primitives to allocate less expensive + memory. They work like kmalloc and kfree versions that give you the right + kind of addresses to store in urb->transfer_buffer and urb->transfer_dma. + You'd also set ``URB_NO_TRANSFER_DMA_MAP`` in urb->transfer_flags:: + + void *usb_alloc_coherent (struct usb_device *dev, size_t size, + int mem_flags, dma_addr_t *dma); + + void usb_free_coherent (struct usb_device *dev, size_t size, + void *addr, dma_addr_t dma); + + Most drivers should **NOT** be using these primitives; they don't need + to use this type of memory ("dma-coherent"), and memory returned from + :c:func:`kmalloc` will work just fine. + + The memory buffer returned is "dma-coherent"; sometimes you might need to + force a consistent memory access ordering by using memory barriers. It's + not using a streaming DMA mapping, so it's good for small transfers on + systems where the I/O would otherwise thrash an IOMMU mapping. (See + Documentation/core-api/dma-api-howto.rst for definitions of "coherent" and + "streaming" DMA mappings.) + + Asking for 1/Nth of a page (as well as asking for N pages) is reasonably + space-efficient. + + On most systems the memory returned will be uncached, because the + semantics of dma-coherent memory require either bypassing CPU caches + or using cache hardware with bus-snooping support. While x86 hardware + has such bus-snooping, many other systems use software to flush cache + lines to prevent DMA conflicts. + +- Devices on some EHCI controllers could handle DMA to/from high memory. + + Unfortunately, the current Linux DMA infrastructure doesn't have a sane + way to expose these capabilities ... and in any case, HIGHMEM is mostly a + design wart specific to x86_32. So your best bet is to ensure you never + pass a highmem buffer into a USB driver. That's easy; it's the default + behavior. Just don't override it; e.g. with ``NETIF_F_HIGHDMA``. + + This may force your callers to do some bounce buffering, copying from + high memory to "normal" DMA memory. If you can come up with a good way + to fix this issue (for x86_32 machines with over 1 GByte of memory), + feel free to submit patches. + +Working with existing buffers +============================= + +Existing buffers aren't usable for DMA without first being mapped into the +DMA address space of the device. However, most buffers passed to your +driver can safely be used with such DMA mapping. (See the first section +of Documentation/core-api/dma-api-howto.rst, titled "What memory is DMA-able?") + +- When you have the scatterlists which have been mapped for the USB controller, + you could use the new ``usb_sg_*()`` calls, which would turn scatterlist + into URBs:: + + int usb_sg_init(struct usb_sg_request *io, struct usb_device *dev, + unsigned pipe, unsigned period, struct scatterlist *sg, + int nents, size_t length, gfp_t mem_flags); + + void usb_sg_wait(struct usb_sg_request *io); + + void usb_sg_cancel(struct usb_sg_request *io); + + When the USB controller doesn't support DMA, the ``usb_sg_init()`` would try + to submit URBs in PIO way as long as the page in scatterlists is not in the + Highmem, which could be very rare in modern architectures. -- cgit v1.3.1